The present invention relates to a parallel circuit including a diode and an insulated gate bipolar transistor (IGBT) as a high-speed switching element, to a module comprising an anti-parallel connection of an IGBT and a diode on a common base thereof, and to an inverter circuit using the module.
With expansion of the fields of application of power inverters, it has been essential to provide a small-sized inverter developing a high performance with a low noise. As a control element of the inverter, an IGBT having a high-speed switching characteristic of a power metal-oxide-semiconductor field-effect transistor (MOS-FET) and a high power characteristic of a bipolar transistor have been recently developed for practical application.
An IGBT has a fast turn-on time and a turn-off time ranging from 0.1 to 0.5 microseconds and hence is suitably driven into operation with a high frequency. In practical applications, the IGBT is connected to a diode in an anti-parallel fashion on a common base thereof. One to several such pairs are used to configure a module, which has been employed in a high-frequency inverter.
However, when such an IGBT is adopted to constitute an inverter system, a large voltage change or oscillation appears in a switching operation to cause a noise, which may possibly lead to a problem of an erroneous operation of the controller and/or a destruction of the IGBT.
A description will now be given of such an erroneous operation appearing when an inverter includes a module in which an IGBT and a diode are connected to each other in an anti-parallel (inverse-parallel) fashion. When a diode is recovered, an oscillating voltage is applied to an IGBT connected in parallel thereto. This causes an improper or erroneous driving circuit including the IGBT and hence a short circuit of the dc supply is formed to allow a large current to flow therethrough. In the vicinity of the critical range of the wrong operation, the oscillating voltage disappears due to the short circuiting of the inverter arm and hence the wrong operation is interrupted, thereby preventing the destruction of the IGBT in some cases. It has been found, however, that when the critical range is exceeded to cause the oscillating voltage, a large short circuit current of the dc supply flows to cause destruction of the IGBT.
As a method of preventing the erroneous operation described above, there have been known, for example, a method in which a gate signal to be supplied to the IGBT is made to gradually rise up to reduce the current change value di/dt at the turn-off time of the IGBT and a method in which the inverter is provided with a main control wiring with a lowered inductance to minimize the value di/dt at the turn-off time of the IGBT. These methods however disadvantageously lower the circuit operation speed, which is a problem that a high-speed circuit cannot be implemented. Namely, these methods are attended with a problem that the high-speed switching characteristic of the IGBT cannot be fully utilized.